In order to effectively wipe a windshield, the wiper blade must maintain contact, under constant or substantially constant pressure, with the windshield being wiped as the wiper blade and the wiper arm which carries the wiper blade oscillate between a horizontal "inwipe" position and a vertical "outwipe" position. Otherwise, portions of the windshield will not be wiped or will not be wiped effectively if the pressure of the wiper blade against the windshield varies. The requirement to maintain the contact of the wiper blade against the windshield at substantially constant pressure is more difficult to satisfy with highly curved and aerodynamic windshields which are very common because of the increasing emphasis on more efficient fuel consumption.
Typically, the wiper arm unit consists of a base which is fixed to an oscillating post of the wiper drive mechanism and a wiper arm which pivots with the base and to which the wiper blade is attached. A tension spring urges the wiper blade against the windshield, so that as the wiper arm and wiper blade oscillate between the inwipe and outwipe positions, the spring expands and contracts and the wiping pressure varies. A compromise value for the wiping pressure is tolerated, higher at some points in the wipe pattern and lower at others.
Another, but less common, construction of the wiper arm unit includes a cantilevered wiper arm which maintains wiping pressure between the wiper blade and the windshield due to the deflection of the arm itself rather than from a separate spring. The same problem still exists, however, because the deflection of the arm will increase and decrease as the arm deflects.